1. Field of the Invention
This invention relates generally to a system and method of defect correction in solid state imagers and, more particularly, to charge coupled device (CCD) and charge injection device (CID) imagers employing a pixel correction circuit with reduced memory requirements.
2. Description of the Prior Art
Charge coupled imaging devices and charge injection imaging devices comprising a plurality of photosensitive elements arranged in a matrix of rows and columns are well known in the art. Each of the photosensitive elements comprises row and column electrodes. All of the row electrodes in each row are in common connection with respect to each other, and all of the column electrodes in each column are in common connection with respect to each other also. Incident scene light operates to photogenerate minority charge carriers in potential wells under each pair of electrodes in each photosensitive element. The photogenerated charges may be transferred out of the imager as a pulse train of analog voltages by well known scanning techniques. The analog signals may thereafter be color or gamma corrected digitized and stored in a buffer memory.
A variety of techniques are known for correcting defects in a video output signal from an image sensing device such as a photosensor array. Matsuoka et al., U.S. Pat. No. 4,701,784, employs complex averaging and correlation circuits in one embodiment for using signals or pixels around the defective one. The patent also mentions known correction methods in which a memory is employed to store the position of defects while image data is used in the correction process.
In Frame et al, U.S. Pat. No. 4,590,520, dead spots within a photo array are detected based upon excessive rate of change in sequentially accessed prestored digital sensitivity correction coefficients corresponding to the array of photosensitive elements. The leading and trailing edges of dead spots are detected by comparing arithmetic differences between successive correction coefficients to predetermined threshold values. In one embodiment the last video signal value is employed to fill in the dead spots.
Bremmer, U.S. Pat. No. 4,802,011, describes a correction circuit in which a defective picture pickup element is replaced by a proceeding, non-defective picture pickup element employing a signal sampling circuit and a real time video signal.
Youse et al., U.S. Pat. No. 4,805,013, produces bad pixel data by exposing the imager to 50% full well. The bad pixel location is stored in a PROM. Circuitry inhibits bad pixel data from being utilized and uses PROM pixel data instead.
Bencuya et al., U.S. Pat. No. 4,843,473, assigned to the assignee herein, describes a charge injection device with low noise readout in which KTC and fixed pattern noise are subtracted from the signals retrieved from the charge injection imaging device.
U.S. Pat. No. 4,734,774 describes a CCD image defect compensation circuit in which adjacent streams of data are compared and corrected by replacement of adjacent data.
Similarly, methods have been devised in which a threshold detector looks at the combined image pulse and the fixed pattern noise (or dark current) of the pixel. If a given threshold is exceeded the pixel information is discarded and some other value is substituted therein. An example of such an approach is shown in Endo et al., U.S. Pat. No. 4,567,525. A problem with this approach is similar to that encountered with frequency sensitive systems in which a sharp transition or increase in the pixel response may be due to a line in the picture as opposed to a bad element.
Various on-line approaches to correct pixel defects have been attempted including approaches in which the frequency or amplitude characteristic of successive pixel signals or pulses is examined. For example, Farnside, U.S. Pat. No. 4,535,359 and Scaggs, U.S. Pat. No. 4,734,774, describe various methods in which the leading and trailing edges of the pulses are sampled. An abrupt change in pulse height may be indicative of a defective pixel. The problem with such approaches is that there may be a contrasting element or an image line in the pixel which results in a sharp change in the pixel intensity. Accordingly, such methods are not always effective if the contrast is very high and the pulse transition is sharp (indicative of a high frequency pulse).
Some of the described prior arrangements employ circuitry which is complicated and requires additional storage devices for handling the defect information. Some arrangements sense the real time image data signals in order to determine whether a defect exists even though image data varies from image scene to image scene. It is thus difficult to correlate or correct for the defects, especially in cases where the defect do not always appear. For example, as scene light changes or as temperature varies.
Therefore, it is a primary object of the invention to provide a defect correcting apparatus for a CID or CCD imaging device in which defect correction is independent of the incident scene light.
It is a further object of the invention to provide a defect correction apparatus requiring reduced memory storage.
Other objects of the invention will be in part obvious and will in part appear hereinafter. The invention accordingly comprises a system possessing the construction, combination of elements and arrangement of parts which are exemplified in the following detailed disclosure.